1. Field of the Invention
This invention relates to a process for operating an internal combustion engine with low pollutant emission.
2. Description of the Prior Art
To preserve a livable environment, it is necessary to reduce the pollutant emission of internal combustion engines as much as possible. Increasingly stringent limits are being prescribed by legislation that limit the fraction of pollutants in exhaust gas. In the past, for example, the exhaust gas limits for motor vehicles (automobiles and commercial vehicles) have been cut in half about every 3 to 5 years.
However, in road traffic itself, where rapid changes of speed and load are expected of the engines, great difficulties are confronted in reducing the emission of pollutants without any adverse impact on road performance, or conversely without having to deal with an increase of fuel consumption.
Corresponding to the importance of irregular operation, a running curve is prescribed for measuring the pollutant emission of automobiles that includes running hot after a cold start and some acceleration phases. For commercial vehicles also, replacing the present steady-state 13-point test with a dynamic running curve is foreseen for the future. A stepwise approximation of the test cycles to actual operation will thus be achieved for both types of motor vehicles.
To evaluate a cycle test, samples of the exhaust gases produced during the entire cycle are collected and only the amount of pollutants integrated over time is considered. A large fraction of the pollutant emissions originates from the warmup phase and from non-steady-state operation of the engine.
It has long been known that the chemical energy contained in the exhaust gas can be utilized by installing a noble metal catalyst in the exhaust line to bring about a post-reaction of the pollutants. When the catalyst reaches a minimum temperature, the unburned hydrocarbons, carbon monoxide, and nitrogen oxides in the exhaust gas react to form harmless compounds. This technique has been used since recently in diesel engine vehicles as a so-called oxidation catalyst and achieves an efficiency of about 30-50% for reduction of particulates, HC, and CO.
Such a small pollutant reduction is inadequate in the case of the gasoline engine, so that this catalyst, called unregulated here, has been replaced by a 3-way catalyst with .lambda.-regulation for about ten years. This can drastically improve pollutant conversion and it achieves an efficiency of far greater than 90%.
During the acceleration phases, however, more fuel would be necessary for ideal operation of the engine, but this would shift the air-to-fuel ratio to values lower than 1 and would lead to poorer catalyst efficiency with regard to HC and CO. Even when the load is eliminated, there is a deviation of the mixture from the ideal point since more fuel is vaporized from the intake manifold walls because of the changed flow in the intake system. The value of .lambda. in non-steady-state operation can be kept roughly constant only by complicated measures.
Another drawback of known catalyst technology is the minimum temperature of the catalyst necessary for the chemical reactions to start. Thus, after a cold start of the engine it takes several minutes for the catalyst material to be heated by contact with the hot exhaust gas. Slightly enriching the mixture during this warmup phase can cause the combustion temperatures in the engine to rise, and the exhaust gas becomes hotter and also contains increased chemical energy in the form of unburned hydrocarbons, which in turn heats the catalyst. Drawbacks to this are increased consumption because of the richer mixture and an increase in the untreated concentration of pollutants, which are unfortunately released to the environment practically without exhaust gas aftertreatment, since the catalyst is in fact still inactive.
Since 60-70% of the pollutants emitted over the entire cycle are typically formed in the first 50 seconds of a test cycle, supplementary measures outside the engine are being tested at this time for heating the catalyst. By moving the exhaust gas catalyst close to the engine, the heat energy contained in the exhaust gas can be better utilized. However, there is a risk that the catalyst will be heated too strongly, damaged, or even destroyed under full load.
Mounting heaters in the catalyst is also being investigated. There are burner-heated systems, but there are substantial safety objections against them. Electrically heated systems, on the other hand, require a more generous design of generator and battery. The latter is problematical in particular because the electrical energy is needed at a time when the motor is producing practically no mechanical energy. Both systems have in common the drawback that the heat capacity of the exhaust system has to be overcome, which in every case leads to the fact that the efficacy of exhaust gas purification can start only with some time delay after turning on the engine.
DE-A-42 31 581 discloses a process with the process steps of the preamble of claim 1. Admixtures affecting conversion are carried out and all operating parameters of the plasma chemical reactions that bring about conversion are controlled to maximize pollutant conversion.
JP-A-07247827 discloses how to operate an electrical exhaust gas aftertreatment system in the exhaust system of a diesel internal combustion engine with low exhaust gas temperature prevailing. The aftertreatment system operates during the entire warmup time and is turned off at high exhaust gas temperature.